power: Support power lock for headless profile

power: Support power lock for headless profile

The functions below have become operational on headless profile.
 - device_power_request_lock()
 - device_power_release_lock()

In addition, the functions below have become working properly as well
on headless profile associated with the functions aboves.
 - device_power_get_lock_count()
 - device_power_add_lock_count_changed_callback()
 - device_power_remove_lock_count_changed_callback()

Note that, device_power_change_state() bypasses power locks. This policy
stands valid on both headed and headless profile. So it is encouraged
that checking POWER_LOCK_CPU count using device_power_get_lock_count()
before calling device_power_change_state() for DEVICE_POWER_STATE_SLEEP.

Change-Id: Iaf4d4cdd0aa02dbd6fabea2096a3a011574f97a3
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>